1. Field of the Invention
The invention relates to wadded-wire contacts, especially such contacts for use in an LGA (Land Grid Array) connector used to couple a circuit board to an electronic chip or multi-chip module.
2. Description of the Related Technology
An LGA connector is used for making contact between a system (circuit) board, having an array of contacts, and a substrate, having a corresponding array of LGA-pad contacts, where each of the LGA contacts is aligned with a respective one of the system board contacts. Various types of connectors are known and used for LGA connectors, including wadded-wire contacts. These wire buttons or wads are placed into through-holes in an insulating carrier (plate or sheet) to form the LGA connector. The wads protrude from each end of the hole in the insulating carrier plate to touch and electrically connect with the contacts above and below the insulating carrier, or with mating electrical circuits.
Recently, in order to increase the number of contacts on the chip or multi-chip module (MCM) with a given contact spacing, the area of the side of the MCM facing the LGA connector insulating carrier has been increased by use of the so-called “shallow grind.” The shallow grind removes less material around the edge of the MCM and therefore increases the thickness of the ground edge which is clamped in a C-ring to mount the MCM. The shallow grind is essential if the known and reliable hermetic sealing system, described below, is still to be used.
FIG. 1 shows a shallow-ground edge. (FIG. 1 is not a “prior art” figure because it depicts the invention, but it also depicts an exemplary environment of the invention, including the shallow-ground edge.) FIG. 1 shows an MCM substrate 10 that is held, by clamping its upper and lower ground edges (described below) between a base ring 22 and an upper plate 24 (the clamp is not illustrated in FIG. 1). The uppermost surface 11 of the substrate 10 makes contact with solder balls 31 on the bottom of a chip 30, which forms a thermal interface with the upper plate 24; meanwhile, on the lower surface of the substrate 10 are a plurality of LGA pads (electrical contact areas) 17, which make contact with a system board 50 underneath through an LGA connector 100 and its contacts 120. As is further described below, the LGA connector 100 comprises a sheet or plate of insulating material 110 (a carrier), usually of a plastic material, with individual metallic contacts 120 making electrical contact from one side of the connector 100 to the other side, at points corresponding to the locations of the pads 17. In this way the pads 17 on the bottom of the substrate 10 are electrically coupled to the system board 50.
To the left of the solder balls 31 in FIG. 1 is a decline 12 which leads to a lower, ground surface 13. Directly below the decline 12 in FIG. 1, on the underside of the substrate 10, is an incline 15 which connects the lower surface 16 of the substrate 10 to a ground surface 14. The small height difference between the lower surface 16 and the ground surface 14 is what defines the “shallow grind.”
FIG. 1 shows that the lower surface of the base ring 22 overlaps the upper surface of the system board 50, so that the height difference of the surfaces 14 and 16 must approximate the thickness of the inwardly-protruding portion of the base ring 22. In the earlier deep grind version (not shown), the incline 15 was longer and farther from the lateral edge of the substrate 10, making for a greater height difference between the surfaces 14 and 16 so that the connector 100 could be relatively thin, but also making for a smaller area of the lower surface 16 and hence a smaller number of LGA pads 17. The use of the illustrated “shallow” grind, as opposed to the “deep” grind, requires that the thickness of the LGA connector 100 be increased from about 0.8 mm to about 2.5 mm, which of course increases the thickness of the carrier 110 and the length of the contacts 120 that pass through the carrier 110.
Between the lower ground surface 13 and the underside of the upper plate 24 is a C-ring 40, which acts as an hermetic seal. Also shown in FIG. 1 are a cushion 41, which acts to distribute the C-ring compression force, and an alignment pin 43 that passes through the carrier plate 110 of the connector 100 into the base ring 22. These represent the conventional known and reliable hermetic sealing system mentioned above.
Although not shown in detail in FIG. 1, the contacts 120 of the connector 100 include wadded-wire portions. Wadded-wire contacts are intrinsically reliable because of the numerous points at which they touch their intended contact surfaces (thought to number three to seven coupling points for each wad), and also themselves (at various points along the length of the wire), which provides multiple current paths, redundancy, and reliability. Statistically, they out-perform other types as to failure rate and signal integrity. However, wadded-wire contacts do not operate as well when they are made long, in part because the axial spring constant drops as the length increases. (This happens by an elementary property of springs; for a spring of constant cross-section, the longer it is the lower the spring constant of the whole spring.) Also, the resistance increases with length, and the resistivity of spring metal is typically significantly higher than the resistance of pure copper.
When wadded-wire contacts need to be long, they are conventionally combined with solid-plunger contacts that take up some of the length so that the wadded-wire wads can stay short. Then, electrical contact is made through the wadded-wire wad or wads and the solid plunger or plungers, which are deployed in series within the through-holes. There are various combinations of usually alternating wads and plungers. An example of this prior-art approach is shown in prior-art FIG. 5, where a solid plunger 270 and a wadded-wire wad 250 are both inserted into a through-hole 212 in a plastic carrier plate 210 of a connector 200. The solid plunger 270 provides length and the wad 250 provides resiliency.
This combined plunger-wad contact has the following drawbacks:
First, there is a decrease in reliability (signal integrity or SI), at least in part because a solid plunger introduces another pair of separable interfaces, causing it to be less reliable, and a change in electrical impedance. When they are arranged in series mechanically and electrically, the reliability of the combination cannot be any higher than that of a single contact. The overall contact failure rate can be approximated as the individual contact failure rate multiplied by the number of interfaces.
Second, the DC resistance increases and causes a voltage drop through the MCM-card assembly. Although the solid plunger 270 has low internal resistance, this resistance is in series with that of the wadded wire and the two resistances are additive.
In addition, the center-to-center spacing may be somewhat higher in the case of plunger-wad combinations like that illustrated in FIG. 5. A wad alone typically has a diameter of 0.020 inches and center spacing of 0.040 inches, while a single-wad, single-plunger combination might have the same diameter but a center spacing of 0.050 inches. The same might be true of a plunger-wad-plunger arrangement or a wad-plunger-wad arrangement.
Thus, there has been a need for a longer contact that retains the advantages of the shorter wadded-wire contact.